Conventional conveyor belts comprise an endless loop of conveyor belt which passes over various rollers. Despite various drive roller, head roller and tail roller configurations the belt essentially consists of two runs which are positioned one above the other and extend between a source location and a destination location. In operation the belt travels in opposite directions on the two runs.
The coal, iron ore, bauxite or similar mineral or other particulate material is loaded onto the upper surface of the upper run at the source location and is discharged from this material conveying surface at the destination location. On the lower (return) run, the material conveying surface forms the lower surface.
The conveyed material which is not discharged at the designation location continues with the belt and is termed "carry over". This carry over material which adheres to the belt, often despite the actions of a scraper blade positioned to scrape the material carrying surface of the belt at the destination location, is subject to all the vibration which the belt on the return run experiences. Accordingly, this carry over material is dislodged from the lower surface of the belt during the return run and falls downwardly to the floor of the mine, or working surface of the stockpiling operation.
Such spillage represents a substantial volume, up to 2%, of the total material conveyed. Accordingly, it is not unknown for the depth of such spillage to reach, and even exceed, the height of idler rollers over which the return run of the belt moves. A substantial cost of any such mining or conveying operation is the cost of removing such belt spillage. In a typical Australian underground coal mine utilising longwall mining techniques, approximately 2 million tonnes (tons) are produced each year. The conveyor line to convey this material often consists of eight individual conveyors with a total transport distance from coal face to stock pile of approximately 8 kms (5 miles). It is not unknown for the cost of returning the spillage to the upper surface of the upper run of the conveyor belt to cost more than the spilled material is worth.
In an effort to reduce the volume of carry over material, the material carrying surface of the conveyor belt is normally scraped. Such scrapers fall into two distinct categories. One category is the light scraper which uses a wire, rubber blade, or similar relatively flexible scraper which is intended primarily to dislodge any loose material from the belt surface. The other category of scraper is a hard scraper having a blade of tungsten carbide, or similar. Although such hard scrapers reduce the volume of carry over material, they also substantially contribute to belt wear, by up to 30% under normal operating conditions. In the event of some malfunction, the hard scraper can tear or rip a belt to an extent that it has to be replaced.
In addition to the above mentioned problems of loss of yield, the cost of cleaning up, and the wear of the belt caused by hard scrapers; the problem of carry back and subsequent belt spillage produces other undesirable side effects. In particular, with coal the presence of fine coal dust on the floor of the mine constitutes a substantial fire hazard. In addition to the belt being progressively buried by carry back material, the carry back material also interferes with the operation of rollers, tail tensioners, and other standard items of conveyor apparatus since the material is continually being showered in fine particulate and generally abrasive material.
The carry back of fine particulate material on the return run of conveyors is a much greater problem to present day conveyor operators than in years past. There are two basic reasons for this. Firstly the speed at which conveyor belts are operated has increased dramatically through improved technology. High belt speeds cause the fine material to vibrate and move through the bed of material carried on the belt thus coming into contact with the belt itself.
Secondly, environmental and health considerations require dust to be suppressed as much as possible. The preferred method of suppressing dust is to use water sprays. As a consequence of this activity, a thick layer of wet and sticky material is formed on the belt. This wet and sticky material is particularly difficult to remove by means of scrapers at high belt speeds but drys on the return run and then falls from the belt. This is a severe problem with long belts.
It is known to attempt to overcome the above mentioned problem by the provision of two belt turnover mechanisms each of which longitudinally twists the conveyor belt. Each belt turnover mechanism is located on the return run, one at the beginning of the return run and the other at the end of the turn run. The first turnover mechanism twists the return run so that the material carrying surface is uppermost on the bulk of the return run length, while the second belt turnover mechanism reverses the return run to its original disposition. This system is costly and mechanically complex thereby making it vulnerable to frequent breakdown and substantially increasing the cost of required maintenance.
The object of this prior art approach is to prevent carry over spilling from the belt other than at the beginning and end of the return runs. This at least reduces the spatial problem of belt spillage but does nothing to reduce the volume of belt spillage or the problem of particulate material raining down on tail rollers, and the like.
Patent novelty searches conducted after the making of the present invention have disclosed some prior art, It is known from German Patent Specification No. 3,225,551 to avoid the cost of a roof to cover an overland conveyor by conveying the material on the upper surface of the lower run and using the return run above to protect the material from the weather. There is no mention of the problem of spillage of the material conveyed.
The abovementioned search also disclosed PCT/SE84/00143 published under No. 85/02831 which is concerned with a grain elevator where grain in a trough is conveyed both by vanes on the underside of the belt which scrap along the trough and also by simply being carried by the upper surface of the lower run of the belt if the trough is overfilled. There is clearly no adhesion between the conveyed grain and the belt, indeed a belt with small openings therethrough is disclosed. Because of the trough, the problem of spillage from the belt does not arise. U.S. Pat. No. 3,219,173 makes a similar disclosure.
U.S. Pat. No. 4,068,755 also disclosed by the search discloses an analogous arrangement for use by slurries within a flume trough. Here the conveyor moves heavy particles within the trough which happen to settle out of the slurry onto the upper surface of the conveyor's lower run. The remainder of the slurry flows under gravity through the flume trough. Again the question of material spilling from the trough does not arise because of the presence of the trough.
It is also known from British Patent No. 474,839 of 1937 disclosed in the abovementioned search to convey coal on the upper surface of the lower run of a conveyor belt. This specification discloses a T-shaped conveyor configuration which is intended to convey coal from opposite arms of the cross piece of the T, and then lead the material to the head of the stem of the T. In order to fabricate the conveyor belt of the cross piece of the T from a single belt, and thereby be able to use a single drive mechanism, a belt configuration is adopted in which on one arm of the cross piece of the T, material is conveyed on the upper surface of the lower run of the belt.
This T-shaped conveyor configuration is essentially impractical in mining in that it makes it extremely difficult for men and equipment to be moved alongside the stem of the T, and then past the cross piece of the T, without either going around one arm of the cross piece, or crossing the belt itself. The specification is entirely silent as to the problems of belt spillage, carry over and the like.